1. Field of the Invention
The present invention relates in general to solar cells, and more particularly to solar cells with front contacts having improved electrical and mechanical characteristics.
2. Description of the Related Art
Conventional gallium arsenide (GaAs) solar cells consist of a semiconductor body having an aluminum gallium arsenide window layer overlying a gallium arsenide emitter layer which in turn overlies a gallium arsenide base (buffer) layer. The window layer and emitter are of either N-type or P-type conductivity while the base is of the opposite conductivity type being either P-type or N-type. An N-P or P-N semiconductor junction lies between the emitter and base layers. The emitter layer has a front light-receiving major surface adjacent the window layer and the base layer a back major surface. When light energy impinges on the front light-receiving surface of the cell, electrons and corresponding holes are created in both the emitter and base. For the most part, because of the presence of the semiconductor junction, electrons will be directed toward one major surface of the cell and holes toward the other major surface resulting in a photo-current density. In a typical P-N junction solar cell, holes move to the front light receiving surface of the cell and electrons toward the back surface. Metallic contacts are attached to the front and back surfaces of the solar cell semiconductor body to collect the electrons at one contact and holes at the other contact.
Great efforts have been exerted to make solar cells more efficient. The electrical contacts on the solar cell affect the overall efficiency of the solar cell, and therefore provide an area of concern to industry. The recombination of electrons or holes at the metal-to-semiconductor body interface and the contact resistance between metal contact and semiconductor body must both be kept to a minimum to achieve a high efficiency solar cell. Additionally, the front electrical contact must provide an area on which external contacts can be fastened by high temperature welding or soldering to form strong interconnections.
For gallium arsenide solar cells, the front contacts include grid lines and an ohmic bar. The grid lines provide the means for collecting the charge carriers from the semiconductor body, and the ohmic bar provides an area upon which electrical wire interconnections from other solar cells can be welded. Traditionally, the grid lines and ohmic bar have been placed directly on the aluminum gallium arsenide window layer. A low resistance contact between the metal contact and window layer, however, is difficult to achieve because the aluminum gallium arsenide has a tendency to oxidize and because of the large barrier height between the metal and the aluminum gallium arsenide.
Alternatively, grooves may be etched through the window layer exposing selective portions of the emitter layer. The grid lines and ohmic bar, made of an alloy including zinc, are disposed in the grooves making direct electrical contact to the emitter layer. Contacts fabricated onto the semiconductor body make low resistance contact to the emitter. However, high temperature welding or soldering of the interconnections to the ohmic bar will cause zinc from the metal contact to diffuse into the emitter layer and semiconductor junction region which degrades the efficiency of the cell.